Water-absorbent resin and production process therefor

ABSTRACT

The present invention provides such a production process that a low-cost water-absorbent resin having excellent quality can be obtained by reasonable steps in aqueous solution polymerization. The production process for a water-absorbent resin comprises a polymerization setp that includes the steps of: supplying an aqueous solution of a water-soluble unsaturated monomer component including a major proportion of acrylic acid and/or its salt into a polymerization vessel causing shearing action; and then carrying out polymerization, involving crosslinking, of the water-soluble unsaturated monomer and at the same time carrying out fine division of the resultant hydrogel; with the production process being characterized in that the aqueous solution of the water-soluble unsaturated monomer component as supplied into the polymerization vessel has a temperature of not lower than 40° C.

TECHNICAL FIELD

[0001] The present invention relates to: a water-absorbent resin, whichis favorably used for various uses, such as sanitary articles (e.g.disposable diapers and sanitary napkins) and water-retaining agents forsoil; and a production process therefor.

BACKGROUND ART

[0002] In recent years, water-absorbent resins are widely utilized forvarious uses, such as sanitary articles (e.g. disposable diapers,sanitary napldns, and incontinent articles for adult) andwater-retaining agents for soil, and are mass-produced and consumed.

[0003] Particularly, in uses of sanitary articles such as disposablediapers, sanitary napkins, and incontinent articles for adult, theamount of the water-absorbent resins as used tends to increase, and theamount of a pulp fiber as used tends to decrease, in order to thin theresultant articles. The water-absorbent resins are desired to have largeabsorption capacity under a load. On the one hand, desired are low-costwater-absorbent resins because the amount of the water-absorbent resinsas used per sheet of sanitary articles is large. Therefore, it isdesired that: the energy consumption and the amount of dischargedmaterials are decreased in production lines of water-absorbent resins,and thereby a reasonable production process is established.

[0004] In order to decrease costs for improving a ratio of performanceof the water-absorbent resins to their costs, various attempts (e.g. amethod which involves carrying out polymerization in a high monomerconcentration; and a polymerization method which involves initiatingpolymerization in high temperature, evaporating water by heat ofpolymerization or heating, and obtaining a dried water-absorbent resinat a stroke) have hitherto been made as a method for carrying outaqueous solution polymerization of a monomer component that is convertedto water-absorbent resins by the polymerization.

[0005] In gazettes of JP-A-071907/1983 (Arakawa Chemical Industries.Ltd.) and JP-A-018712/1984 (Arakawa Chemical Industries. Ltd.),disclosed is a method which involves polymerizing an aqueous acrylatesalt solution having a high concentration of higher than 55 weight %,and obtaining a dried solid water-absorbent resin at a stroke. In aspecification of U.S. Pat. No. 4,985,518 (American Colloid), disclosedis a method which involves polymerizing an aqueous acrylate saltsolution having a high concentration of higher than 30 weight %, andobtaining a dried solid water-absorbent resin at a stroke. In a gazetteof JP-A-058208/1980 (Kitani), disclosed is a method that involvescarrying out polymerization without using a crosslinking agent in thepolymerization temperature range of 106 to 160° C. As is shown in itsexample, it is disclosed that a dried solid having a low water contentis obtained when the polymerization is completed. In a gazette ofJP-A-318022/1989 (Mitsubishi Petrochemical Co., Ltd.), disclosed is amethod which involves polymerizing an aqueous solution including amonomer having a neutralization ratio of 20 to 50 mol % in an amount of45 to 80 weight %, and obtaining a polymerized product nearly in a drystate. However, these methods have demerits such that the extractablecontent of formed water-absorbent resins is relatively large incomparison with their absorption capacity.

[0006] In addition, in gazettes of JP-A-147512/1980 (Sumitomo ChemicalCo., Ltd.), JP-A-147809/1981 (Sumitomo Chemical Co., Ltd.),JP-A-275607/1988 (Sanyo Chemical Industries, Ltd.), and JP-A-275608/1988(Sanyo Chemical Industries, Ltd.), it is disclosed that: a dried productof water-absorbent resin is obtained at a stroke by supplying an aqueousmonomer solution onto a heated rotation drum, and then scratching andcollecting the resultant product In a gazette of JP-A-165610/1989 (Rohmand Haas Company), it is disclosed that a substantially dried solidwater-absorbent resin is also obtained by supplying an aqueous monomersolution onto a heated face in the nearly same way. However, thesemethods also have demerits such that the extractable content of formedwater-absorbent resins is relatively large in comparison with theirabsorption capacity.

[0007] In addition, in a gazette of JP-A-215801/1990 (MitsubishiPetrochemical Co., Ltd.), it is disclosed that: the heat ofneutralization of a monomer is utilized, and then the polymerization iscarried out by spraying a heated aqueous monomer solution in a gasphase. However, the control of the polymerization is thought to bedifficult because the polymerization is completed within about 3seconds.

[0008] The above prior arts were techniques as disclosed before 1990,but each has demerits. Therefore, it seems that they are not carried outactually.

[0009] Thereafter, disclosed are techniques for enhancing performance inorder to improve a ratio of performance of the water-absorbent resins totheir costs. In gazettes of JP-A-175319/1992 (Sanyo Chemical Industries,Ltd.) and JP-A-181005/1999 (Nippon Shokubai Co., Ltd.), it is disclosedthat: an attempt is made to obtain water-absorbent resins having highperformance by initiating polymerization at a low temperature, mildlycarrying out the polymerization while being cooled, and therebysuppressing the peak temperature to not higher than about 90° C. In agazette of JP-A-228604/1999 (Nippon Shokubai Co., Ltd.), it is disclosedthat: an attempt is made to obtain water-absorbent resins having highperformance by also initiating polymerization at a low temperature,mildly carrying out the polymerization while being cooled, and therebysuppressing the peak temperature to not higher than about 95° C. orcontrolling the amount of the solid component concentration as increasedin the range of 0.2 to 10 weight %. In addition, in WO 01/38402A1(BASF), it is disclosed that: an attempt is made to obtainwater-absorbent resins having high performance by also initiatingpolymerization at a low temperature, and cooling from a polymerizationvessel wall and discharging the resultant polymerized gel in order tosuppress the consumption of heat of reaction (e.g. in order to suppressthe peak temperature to not higher than about 100° C.). In a-gazette ofJP-A-067404/1997 (BASF) and a specification of U.S. Pat. No. 6,187,828(BASF), disclosed is a method that involves initiating polymerization ata low temperature in a cylindrical polymerization vessel, and thenadiabatically carrying out the polymerization. However, the cooling isnot carried out, and therefore the method has demerits such that theconcentration of the aqueous monomer solution cannot be increased,namely, demerits such that the residence time is prolonged (a fewhours). Both of these are carried out at the sacrifice of productivity,and therefore it is inevitable to need great costs.

[0010] In addition, recently, in Journal of Applied Polymer Science,Vol. 74, 119 to 124 (1999), report was “An Efficient Preparation Methodfor Superabsorbent Polymers” (Chen. Zhao). This proposes a low-costpolymerization method which involves charging an aqueous solution havinga monomer concentration of 43.6% and an initiator in astainless-steel-made petri dish, immersing the petri dish in a waterbath of 70 or 80 ° C., and then carrying out polymerization, but themethod has not reached an industrially useful level.

[0011] In addition, in a gazette of JP-A-045812/1998 (Selisui PlasticsCo., Ltd.), it is disclosed that an attempt is made to prevent bumping,to improve emission of vapor, and to lower the water content of formedgel, by adding a short fiber to an aqueous monomer solution. However, ithas demerits of using the valuable short fiber that does not contributeto water absorption.

DISCLOSURE OF THE INVENTION OBJECT OF THE INVENTION

[0012] An object of the present invention is to provide a process forproducing a water-absorbent resin having excellent performance at lowcost. More particularly, the object is to provide: a base polymer, whichdisplays high absorption capacity without load and has a smallextractable content; and a water-absorbent resin, which issurface-crosslinking-treated and displays high absorption capacity undera load, by reasonable steps.

SUMMARY OF THE INVENTION

[0013] The present inventors diligently studied in order to attain theabove-mentioned object. As a result, they have found out that: contraryto the prior established theory (that water-absorbent resins having highperformance are obtained by initiating polymerization at a lowtemperature, and cooling in order to lower the peak temperature to theutmost, as disclosed in gazettes of JP-A-175319/1992 (Sanyo ChemicalIndustries, Ltd.), JP-A-181005/1999 (Nippon Shokubai Co., Ltd.), andJP-A-228604/1999 (Nippon Shokubai Co., Ltd.)), water-absorbent resinshaving high performance are obtained with high productivity by aninnovative method in view of the prior theory, which method involvesobtaining a hydrogel having a high solid component concentration in ashort time surprisingly by raising the temperature of an aqueous monomersolution as supplied into a polymerization vessel having shear force(causing shearing action), and evaporating water at a boilingtemperature of water in the gel. Then, the present invention has beencompleted. Herein, the term “hydrogel” means a water-absorbent resinhaving a solid component concentration of not more than 82 weight (mass)%.

[0014] In addition, in the production process for a water-absorbentresin, it is important how a hydrogel being formable in thepolymerization and having a high solid component concentration of 55 to82 weight % can be disintegrated. When a hydrogel, as formed by carryingout aqueous solution polymerization of a monomer component that isconverted to water-absorbent resins by the polymerization, has such ashape (e.g. thick plate, block, and sheet) as is difficult to dry as itis, the hydrogel is usually disintegrated, and thereafterwater-absorbent resin products are produced through each step of such asdrying, pulverization, classification, and surface treatment. In thecase of a water-absorbent resin of an acrylic acid (salt) type, ahydrogel can easily be disintegrated with such as a meat-chopper-typedisintegrator when the hydrogel has a solid component concentration ofless than 55 weight %. In addition, when the solid componentconcentration is more than 82 weight %, the hydrogel can easily bepulverized with such as a conventional shock-type pulverizer in the sameway as of a dried polymer. However, the hydrogel having a solidcomponent concentration of 55 to 82 weight % is difficult to handlebecause of its properties, and therefore an attempt to industriallydisintegrate the hydrogel has not been successful yet.

[0015] In such as Comparative Examples 1 and 2 of a specification ofU.S. Pat. No. 4,703,067 (American Colloid), hydrogels having solidcomponent concentrations of 58 % and 67 % respectively were obtained,but it is described therein as follows: “They could not be pulverized asthey were, and it was necessary to dry them before the pulverization.”,and the disintegration in the above solid component concentration rangeis avoided.

[0016] Examples of disintegrators for gels are described in a gazette ofJP-A-175319/1992 (Sanyo Chemical Industries, Ltd.). The polymerizationis carried out in a monomer concentration of 50 weight % at the maximum,but shown is no disintegration example of a hydrogel having a solidcomponent concentration of not less than 55 weight %.

[0017] In gazettes of JP-A-119042/1998 (Nippon Shokubai Co., Ltd.),JP-A-188725/1999 (Nippon Shokubai Co., Ltd.), JP-A-188726/1999 (NipponShokubai Co., Ltd.), it is disclosed that gels are disintegrated byshearing with fixed and rotary cutters, but shown is no disintegrationexample of a hydrogel having a solid component concentration of not lessthan 55 weight %, either.

[0018] In a gazette of JP-A-188727/1999 (Nippon Shokubai Co., Ltd.,invented by Mr. Hatsuda, Mr. Miyake, and Mr. Yano), it is disclosedthat: a hydrogel is sheared by holding it between a pair of spiralrotary cutters which are settled facing each other and have differentadvancing speeds, and then disintegrated. In its Example 1, a hydrogelhaving a water content of 39 weight % is disintegrated, but there is nodisintegration example of a hydrogel having a weight-average particlediameter of not larger than 100 mm. Actually, the weight-averageparticle diameter of the hydrogel as disintegrated is larger than 100mm.

[0019] Accordingly, the present inventors diligently studied the abovematters. As a result, they have established a production process for awater-absorbent resin by applying an aqueous monomer solution having atemperature of not lower than a specific temperature to a specificpolymerization method, which process does not need the disintegrationmethods for a hydrogel as mentioned above.

[0020] That is to say, a production process for a water-absorbent resin,according to the present invention, comprises a polymerization step thatincludes the steps of: supplying an aqueous solution of a water-solubleunsaturated monomer component including a major proportion of acrylicacid and/or its salt into a polymerization vessel causing shearingaction; and then carrying out polymerization, involving crosslinking, ofthe water-soluble unsaturated monomer and at the same time carrying outfine division of the resultant hydrogel; with the production processbeing characterized in that the aqueous solution of the water-solubleunsaturated monomer component as supplied into the polymerization vesselhas a temperature of not lower than 40° C.

[0021] In addition, a water-absorbent resin, according to the presentinvention, is a water-absorbent resin which is obtained by the presentinvention production process, and which displays an absorption capacityof not less than 20 g/g under a load.

[0022] In addition, a sanitary article, according to the presentinvention, comprises the present invention water-absorbent resin.

DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, the modes for carrying out the present invention areexplained in detail.

[0024] Examples of the water-soluble unsaturated monomer component asused in the present invention include: anionic unsaturated monomers,such as (meth)acrylic acid, maleic acid or maleic anhydride, itaconicacid, cinnamic acid, vinylsulfonic acid, allyltoluenesulfonic acid,vinyltoluenesulfonic acid, styrenesulfonic acid,2-(meth)acrylamido-2-methylpropanesulfonic acid,2-(meth)acryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonicacid, 2-hydroxyethyl (meth)acryloyl phosphate, and salts thereof;mercaptane-group-containing unsaturated monomers;phenolic-hydroxyl-group-containing unsaturated monomers;amide-group-containing unsaturated monomers, such as (meth)acrylamide,N-ethyl(meth)acrylamide, and N,N-dimethyl(meth)acrylamide;amino-group-containing unsaturated monomers, such as N,Ndiethylaminoethyl (meth)acrylate, N,N dimethylaminopropyl(meth)acrylate, and N,N-dimethylaminopropyl (meth)acrylamide.

[0025] These monomers may be used either alone respectively or fitly incombinations with each other. However, in view of performance and costsof the water-absorbent resin as obtained, it is necessary to use acrylicacid and/or its salts (e.g. salts of such as sodium, lithium, potassium,ammonium, and amines; of the above, its sodium salt is favorable in viewof costs) as main components. The ratio of the acrylic acid and/or itssalts is favorably not less than 70 mol %, more favorably not less than80 mol %, still more favorably not less than 90 mol %, particularlyfavorably not less than 95 mol %, relative to the entire monomercomponent The solid component concentration in the aqueous solution ofthe water-soluble unsaturated monomer component as mentioned herein maycontain the unsaturated monomer component such as the acrylic acidand/or its salts, and the internal-crosslinking agent, and besides,other additives such as polymerization initiators as mentioned below.

[0026] Publicly hitherto known internal-crosslinking agents can be usedas the above internal-crosslinking agent. Specific examples includeinternal-crosslinking agents as described in page 4 of a gazette ofJP-A-182750/1998. In consideration of reactivity, one kind or at leasttwo kinds of these can be used. Of the above, it is favorable toessentially use a compound having at least two polymerizable unsaturatedgroups. The amount of these as used can fitly be determined dependingupon desired properties of the water-absorbent resin.

[0027] There is no especial limitation on the concentration of thewater-soluble unsaturated monomer component in the aforementionedaqueous solution, but the concentration is favorably not less than 30weight %, more favorably not less than 35 weight %, still more favorablynot less than 40 weight %, still more favorably not less than 45 weight%, still more favorably not less than 50 weight %, still more favorablynot less than 55 weight %, still more favorably in the range of 30 to 70weight %, still more favorably 35 to 60 weight %, still more favorably40 to 60 weight %. In the case where the concentration is less than 30weight %, the productivity is low. In the case where the concentrationis more than 70 weight %, the absorption capacity is lowered.

[0028] When an acid-group-containing monomer is used, there is noespecial limitation on its neutralization ratio. However, it is alsoconsidered that the neutralization after the polymerization is favorablyunnecessary in uses possibly contacting with human bodies, such assanitary articles, and therefore the neutralization ratio is favorablynot less than 50 mol %, more favorably in the range of 50 to 80 mol %(excluding 80 mol %), still more favorably 55 to 78 mol %, mostfavorably 60 to 75 mol %.

[0029] When the acrylic acid is neutralized with an alkali and thenused, it is favorable to effectively utilize heat of neutralizationand/or heat of dissolution (of the acrylic acid and the alkali) forheating to raise the temperature of the aqueous solution of thewater-soluble unsaturated monomer component. In a favorable mode forcarrying out the invention, the polymerization is initiated by adding acrosslinking agent and an initiator to an aqueous solution of thewater-soluble unsaturated monomer component, of which the temperature israised by the neutralization in an adiabatic state, or as is mentionedbelow, the heat of neutralization and/or the heat of dissolution (of theacrylic acid and the alkali) is utilized to remove dissolved oxygen.

[0030] When the polymerization is carried out, hydrophilic polymers(e.g. starch, derivatives of starch, cellulose, derivatives ofcellulose, polyvinyl alcohol, poly(acrylic acid .(salts)), andcrosslinked products of poly(acrylic acid (salts))), chain transferagents (e.g. hypophosphorous acid (salts)), and chelating agents mayalso be added to the reaction system.

[0031] As to the polymerization method for the above monomer component,used is a polymerization step that includes the steps of: supplying anaqueous solution of a water-soluble unsaturated monomer componentincluding a major proportion of acrylic acid and/or its salt into apolymerization vessel causing shearing action (having shear force); andthen carrying out polymerization, involving crosslinking, of thewater-soluble unsaturated monomer and at the same time carrying out finedivision of the resultant hydrogel Such the polymerization method iscarried out, and therefore the hydrogel as discharged from thepolymerization vessel can be introduced into a drying step as it is, andsurplus facilities (for the fine division of the gel) such as hydrogeldisintegrators are especially unnecessary, and the polymerization can becarried out very reasonably and at low cost

[0032] As to the polymerization vessel causing shearing action (havingshear force) as used herein, even single-shaft agitation machines can beused, but agitation machines having at least two agitation shafts (e.g.twin-arm kneaders) are favorably used. More favorably used arepolymerization vessels having a rotation agitation shaft, in which thecontinuous polymerization that involves continuously supplying theaqueous solution of the water-soluble unsaturated monomer component andcontinuously discharging the resultant hydrogel can be carried out.Particularly favorably used are polymerization vessels having at leasttwo rotation agitation shafts. Examples thereof include triple-shaftkneaders (kneader-ruders) having two agitation paddles and onedischarging screw, and twin-shaft-extruding kneaders or blenders. Of theabove, the most favorable polymerization vessels for obtainingwater-absorbent resins having high performance with high productivityare continuous kneaders having piston flowability, which have tworotation agitation shafts in such a manner that the aqueous solution ofthe water-soluble unsaturated monomer component is continuously suppliedinto the polymerization vessel and the resultant hydrogel iscontinuously discharged. In the case of using the aforementionedcontinuous kneaders, the productivity is greatly improved in comparisonwith batchwise polymerization vessels. When the polymerization iscarried out by supplying the aqueous solution of the water-solubleunsaturated monomer component into such as a batchwise polymerizationvessel, the polymerization reaction in the present invention proceedsviolently while being vapored and swollen. Therefore, from the viewpointof safety, the amount of the aqueous solution of the monomer componentas supplied cannot help being reduced. In addition, time is alsonecessary to discharge the hydrogel. On the other hand, in the case ofusing the continuous kneaders as the polymerization vessels, the aqueoussolution of the monomer component can be supplied continuously, and theresultant hydrogel is continuously discharged. Therefore, the highproductivity is obtained.

[0033] Furthermore, in order to prevent such as adhesion of uselessgels, the surface roughness of the inner faces of these polymerizationvessels is favorably reduced by resin-coating with such as Teflon(registered trade mark) or by electrolysis grinding, and polymerizationvessels having stainless-steel-made inner faces are particularlyfavorably used. Furthermore, the polymerization vessel is favorablycooled or heated with a jacket from the outside, and further theagitation paddles themselves are also favorably equipped with a coolingor heating structure by installing medium passageways therein. Inaddition, the volume of the polymerization vessel is fitly determined,and is usually favorably in the range of 0.001 to 10m³, and the aqueoussolution of the monomer component is favorably charged in an amount of10 to 90%, more favorably 20 to 70%, relative to the volume.

[0034] In addition, the rotation agitation shafts existing in thesepolymerization vessels are rotated for at least a predetermined timeduring the polymerization, and then the fine division of the hydrogel iscarried out. The rotation speed may be constant or changeable, or therotation may be stopped temporarily or intermittently. Specifically, thestatic polymerization and the rotation polymerization (shearpolymerization) may be carried out together in the polymerization vesselcausing shearing action (having shear force). Furthermore, when at leasttwo agitation shafts are used, these agitation shafts may be rotated inthe same direction or in different directions, but at least twoagitation shafts are favorably in the different directions toward theinside. In addition, the rotation speeds of both may be identical ordifferent.

[0035] Specific examples of the polymerization vessels causing shearingaction (having shear force) are as follows:

[0036] Twin-arm kneader (KNEADER, Kurimoto, Ltd.);

[0037] Twin-arm kneader-ruder (KNEADER-RUDER, Moriyama Co., Ltd.);

[0038] Continuous kneader (CONTINUOUS KNEADER, Dalton Co., Ltd.);

[0039] Paddle dryer (PADDLE DRYER, Nara Kikai Seisakusho Co., Ltd.);

[0040] KRC kneader (KURIMOTO-READCO CONTINUOUS KNEADER, Kurimoto, Ltd.);

[0041] Extruder (EXTRUDER, Kurimoto, Ltd.);

[0042] Honda De-Airing Extruder (HONDA DE-AIRING EXTRUDER, Honda TekkoCo., Ltd.); Chopper (CHOPPER, Hiraga Kosakusho Co., Ltd.);

[0043] Twin· Dome Gran (TWIN· DOME GRAN, Fuji Powdal Co., Ltd.); and

[0044] Bivolak (BIVOLAK, Sumitomo Heavy Industries. Ltd.).

[0045] In radical aqueous solution polymerization, the dissolved oxygenthat inhibits the polymerization is generally removed by blowing inertgas or by carrying out deaeration under a reduced pressure before theaddition of the polymerization initiator. However, it costs expenses forfacilities and operation therefor in actual circumstances. In thefavorable mode for carrying out the present invention, the operation ofremoving the dissolved oxygen is carried out by utilizing the heat ofneutralization and/or the heat of dissolution (hydration) (of theacrylic acid and the alkali) as mentioned above, and heating to raisethe temperature of the aqueous solution of the monomer component, andthereby vaporizing the dissolved oxygen out.

[0046] In more favorable mode for carrying out the invention, thetemperatures of such as acrylic acid, an aqueous alkali solution, andwater as raw materials for the aqueous solution of the monomer componentare raised by the neutralization without beforehand deoxidization, andthe amount of the dissolved oxygen is favorably adjusted to not largerthan 4 ppm, more favorably not larger than 2 ppm, most favorably notlarger than 1 ppm, relative to the aqueous solution of the monomercomponent, and then they are subjected to the polymerization without thedeoxidization operation as they are.

[0047] In addition, it is also favorable that: some or all of such asthe acrylic acid, aqueous alkali solution, and water as raw materialsfor the aqueous solution of the monomer component are beforehandpartially deoxidized, and they are further deoxidized by the rise of thetemperature due to the neutralization. In addition, when thepolymerization is initiated at a high temperature of not lower than 80°C. by carrying out line-mixing neutralization of the acrylic acid andthe alkali and further line-mixing a polymerization initiator, it isfavorable that the deoxidization of such as the raw acrylic acid,aqueous alkali solution, and water is not carried out beforehand inorder to prevent the polymerization from initiating in the line.

[0048] The polymerization is usually carried out under an atmosphericpressure, but it is also a favorable mode that the polymerization iscarried out under a reduced pressure with distilling off water in orderto lower the boiling temperature of the polymerization system. Thepolymerization is more favorably carried out under an atmosphericpressure because of such as easiness of operation.

[0049] There is no especial limitation on the increase of theneutralization ratio during the polymerization, but the increase of theneutralization ratio is favorably not less than 2 mol %, more favorablynot less than 3 mol %, still more favorably not less than 4 mol %. Evenif the increase of the neutralization ratio is zero, there is noespecial problem. However, in the case where the increase of theneutralization ratio is not less than 2 mol %, there are advantages inthat the properties of the polymer as obtained (e.g. hydrogel, basepolymer, or water-absorbent resin) are improved.

[0050] There is no especial limitation on the polymerization initiatorusable in the present invention. Usable are such as pyrolysis-typeinitiators (e.g. persulfate salts, such as sodium persulfate, potassiumpersulfate, and ammonium persulfate; peroxides, such as hydrogenperoxide, t-butyl hydroperoxide, and methyl ethyl ketone peroxide; andazo compounds, such as azonitrile compounds, azoamidine compounds,cyclic azoamidine compounds, azoamide compounds, alkylazo compounds,2,2′-azobis(2-amidinopropane) dihydrochloride, and2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride)) andphotolysis-type initiators (e.g. benzoin derivatives, benzylderivatives, acetophenone derivatives, benzophenone derivatives, and azocompounds). The persulfate salts are favorable in view of ability tolower costs and residual monomers. In addition, the use of thephotolysis-type initiator and ultraviolet ray is also a favorablemethod, and it is more favorable to use the photolysis-type initiatorand the pyrolysis-type initiator together.

[0051] It is favorable to beforehand raise the temperature of theaqueous solution of the monomer component as supplied into thepolymerization vessel. The reason is because the aforementioneddissolved oxygen is easily removed by doing such, and also because thefavorable polymerization initiation temperature as mentioned below canbe realized at once. There is no especial limitation on such thetemperature of the aqueous solution of the monomer component, but thetemperature is usually not lower than 40° C., favorably not lower than50° C., more favorably not lower than 60° C., more favorably not lowerthan 70° C., more favorably not lower than 80° C., more favorably notlower than 90° C., more favorably in the range of 80 to 105° C., mostfavorably 90 to 100° C. In the case where the temperature is lower than40° C., not only the productivity is lowered because of prolonging theinduction period and polymerization time, but also the properties of thewater-absorbent resin are deteriorated. Incidentally, the inductionperiod means how long time passes since the aqueous solution of themonomer component and the aqueous initiator solution are blended untilthe polymerization is initiated, and the polymerization time means howlong time passes since the aqueous solution of the monomer component issupplied into the polymerization vessel until the resultant hydrogel isdischarged from the polymerization vessel.

[0052] Incidentally, in order to ensure the temperature of this aqueoussolution of the monomer component and to cause the polymerizationinitiation, as is mentioned above, the heat of neutralization and/or theheat of dissolution (of the acrylic acid and the alkali) of the aqueoussolution of the monomer component are favorably utilized.

[0053] In addition, it is favorable to introduce the initiator into theaqueous solution of the monomer component as supplied into thepolymerization vessel. The reason is because: when the temperature ofthe aqueous solution of the monomer component is, for example, adjustedto not lower than 40° C. in the polymerization vessel, thepolymerization is initiated as soon as the initiator is added thereto,and the mode of the polymerization seems to be the same mode as of thepresent invention in appearance, but the polymerization proceeds so fastthat the blending of the polymerization initiator is insufficient(non-uniform), and the high extractable content is caused. Furthermore,there is also a problem such that the polymerization is initiated whilethe aqueous solution of the monomer component is heated (before theaddition of the initiator). In this connection, when the temperature ofthe aqueous solution of the monomer component is lower than 40° C., themode of the polymerization is not different from conventional modes inany way, and the polymerization does not attain such high productivityand high performance as obtained in the present invention.

[0054] There is no especial limitation on the highest temperature of thehydrogel in the polymerization vessel. Its minimum value is favorablynot lower than 100° C., and its maximum value is favorably not higherthan 150° C., more favorably not higher than 140° C., more favorably nothigher than 130° C., more favorably not higher than 120° C., morefavorably not higher than 115° C. The highest temperature is mostfavorably in the range of 100 to 115° C. In the case where the highesttemperature is higher than 150° C., there are disadvantages in that theproperties of the polymer as obtained (e.g. hydrogel, base polymer, orwater-absorbent resin) are extremely deteriorated.

[0055] In the present invention, the difference ΔT between thetemperature of the aqueous solution of the monomer component as suppliedinto the polymerization vessel and the highest temperature of thehydrogel in the polymerization vessel is favorably not higher than 70°C., more favorably not higher than 60° C., still more favorably nothigher than 50° C., still more favorably not higher than 40° C., stillmore favorably not higher than 30° C., most favorably not higher than25° C. In the case where the ΔT is higher than 70° C., there aredisadvantages in that the properties of the polymer as obtained (e.g.hydrogel, base polymer, or water-absorbent resin) are deteriorated.

[0056] There is no especial limitation on the polymerization time, butit is favorably not more than 10 minutes, more favorably not more than 5minutes, more favorably less than 5 minutes, more favorably not morethan 3 minutes. In the case where the polymerization time is more than10 minutes, there are disadvantages in that the productivity of thepolymer as obtained (e.g. hydrogel, base polymer, or water-absorbentresin) is lowered.

[0057] Thus, in the present invention, the induction period and thepolymerization time are shortened very much, and therefore it is alsoeasy to add a fine powder of at least one member selected from the groupconsisting of a hydrogel, a base polymer and a surface-crosslinkedwater-absorbent resin as formed together with the production, either tothe aqueous solution of the monomer component or at the same time as thesupply of the aqueous solution of the monomer component. Incidentally,herein, a water-absorbent resin fine powder contains powders passingthrough a sieve having a mesh opening size of 150 μm in not less than 70weight %. In this connection, when the fine powder is added to theaqueous solution of the monomer component in the hitherto polymerizationas initiated at a low temperature, the polymerization time is long.Therefore, there are problems such that: the monomer is absorbed in thefine powder, and the residual monomer is increased, and the absorptioncapacity is difficult to adjust. Even if they are added thereto, amethod such that the amount as added is decreased in order to preservethe properties has been carried out.

[0058] According to favorable examples of the polymerization method inthe present invention, the polymerization goes on, while the temperatureof the system rapidly rises after the initiation of the polymerizationand reaches a boiling point with a low polymerization conversion of suchas 10 to 20 mol %, and water vapor is generated, and the solid componentconcentration is increased. The heat of the polymerization iseffectively utilized, and the solid component concentration isincreased. Therefore, it is desirable to suppress heat radiation fromconnecting material portions of the polymerization vessel to the utmost.Favorable used are such as polymerization vessels in which thenon-connecting material portions. (comprised of resins, rubbers, orstainless steel as the material quality) are covered with heat-retainingmaterials, or polymerization vessels as heated with jackets. The watervapor as generated from the system may contain the monomer, andtherefore it is desirable to recover and reuse (recycle) it in such acase. Particularly, it is favorable that the acrylic acid and/or waterhaving vaporized during the polymerization are collected and then reused(recycled). The recovery ratio of the acrylic acid is favorably not lessthan 1%, more favorably not less than 2%, still more favorably not lessthan 3%, of the weight of the entire acrylic acid (before theneutralization) as used.

[0059] Incidentally, as to the recovery method for the water vapor asgenerated from the system, effective is a method that involvesrecovering the water vapor while introducing a gas into thepolymerization vessel or aspirating the water vapor therefrom. Inaddition, in the case of introducing the above gas, there are many caseswhere inert gas containing no oxygen is generally used in order todecrease the residual monomer in the hydrogel. However, the presentinvention is characterized by enabling the polymerization time toshorten, and by enabling to obtain a water-absorbent resin, in which theamount of the residual monomer is little because the water vapor hasbeen generated severely from the hydrogel during the polymerization evenif oxygen-containing gas may be used.

[0060] In addition, the present invention process is characterized inthat the polymerization is carried out at a high temperature from theinitiation of the polymerization, and the above characteristic seems tocause high performance. In the polymerization under an atmosphericpressure, a favorable mode is the polymerization such that: thetemperature has already been not lower than 100° C. when thepolymerization conversion is 40 mol %, and the temperature has also beennot lower than 100° C. when the polymerization conversion is 50 mol %. Amore favorable mode is the polymerization such that: the temperature hasalready been not lower than 100° C. when the polymerization conversionis 30 mol %, and the temperature has also been not lower than 100° C.when the polymerization conversion is 50 mol %. The most favorable modeis the polymerization such that: the temperature has already been notlower than 100° C. when the polymerization conversion is 20 mol %, andthe temperature has also been not lower than 100° C. when thepolymerization conversion is 50 mol %. In the polymerization under areduced pressure, a favorable mode is the polymerization such that: thetemperature has already reached a boiling temperature when thepolymerization conversion is 40 mol %, and the temperature has also beenthe boiling temperature when the polymerization conversion is 50 mol %.A more favorable mode is the polymerization such that: the temperaturehas already reached a boiling temperature when the polymerizationconversion is 30 mol %, and the temperature has also been the boilingtemperature when the polymerization conversion is 50 mol %. The mostfavorable mode is the polymerization such that: the temperature hasalready reached a boiling temperature when the polymerization conversionis 20 mol %, and the temperature has also been the boiling temperaturewhen the polymerization conversion is 50 mol %.

[0061] In this way, the temperature is high with a low polymerizationconversion, and therefore the polymerization time as required is alsoshort, and the polymerization is usually finished within 10 minutes,more favorably within 5 minutes. Herein, the polymerization time asrequired means how long time passes since the aqueous solution of themonomer component as obtained by adding the polymerization initiator issupplied into the polymerization vessel until the resultant hydrogel isdischarged from the polymerization vessel.

[0062] In addition, as to the timing of the addition of the initiator,the aqueous solution of the monomer component and the aqueous initiatorsolution may be blended in the polymerization vessel, but it isfavorable that the aqueous solution of the monomer component and theaqueous initiator solution are blended before they are supplied into thepolymerization vessel. There is a more favorable method in which theaqueous solution of the monomer component and the aqueous initiatorsolution are blended just before they are supplied into thepolymerization vessel.

[0063] In addition, in order to prevent the initiation of thepolymerization and the clogging of such as supplying lines beforesupplying a mixed solution of the aqueous solution of the monomercomponent and the aqueous initiator solution into the polymerizationvessel, such as polymerization inhibitors can also be made to exist inthe monomer. There is no especial limitation on the polymerizationinhibitor, and effective are such as: o-, m-, or p-methoxyphenol, andmethoxyphenols further having one or at least two substitutent groups(e.g. a methyl group, a t-butyl group, and a hydroxyl group)(particularly favorably p-methoxyphenol); and other hydroquinones,copper salts, and methylene blue. They may be used either alonerespectively or in combinations with each other. It is favorable thatthe polymerization inhibitor is usually made to exist in acrylic acid asused for preparing the aqueous solution of the monomer component.Specifically, in the present invention, one of the favorable modes is touse acrylic acid containing p-methoxyphenol as the water-solubleunsaturated monomer component. The amount of the polymerizationinhibitor existing in the above acrylic acid is favorably not largerthan 200 weight ppm, more favorably in the range of 10 to 160 weightppm. In addition, also exemplified is a method that involves delayingthe polymerization initiation time without entirely removing such asfurfural in a purification step for producing the above acrylic acid.

[0064] In the present invention, it is desirable to carry out thepolymerization while water is vaporized in such a manner that: theconcentration ratio, which is defined as a ratio of a solid componentconcentration of the hydrogel as discharged from the polymerizationvessel to a solid component concentration of the aqueous solution of themonomer component as supplied into the polymerization vessel, isfavorably not less than 1.10, more favorably not less than 1.15, stillmore favorably not less than 1.20, particularly favorably not less than1.25. In the case where the concentration ratio is less than 1.10, itcannot be said that the utilization of the heat of the polymerization issufficient. Herein, the solid component of the aqueous solution of themonomer component means the monomer and other additives, and does notinclude water and solvents.

[0065] As to the solid component concentration of the hydrogel asobtained by the present invention, its maximum value is favorably notmore than 82 weight %, more favorably not more than 75 weight %, and itsminimum value is favorably not less than 50 weight %, more favorably notless than 55 weight %. In addition, the solid component concentration isfavorably in the range of 50 to 80 weight %, more favorably 55 to 75weight %. In the case where this solid component concentration is morethan 82 weight %, observed is the deterioration of performance, namely,the lowering of absorption capacity and the increase of extractablecontent In addition, in the case where the solid component concentrationis less than 50 weight %, there are disadvantages in that the drying ina following step bears a heavy burden.

[0066] The above hydrogel favorably has a form that is formed by foamingexpansion and contraction during the polymerization. This is a form madeby a manner that: the polymerization system is foamed to have a foamdiameter of several millimeters to several centimeters in unit by thewater vapor pressure as caused by the boiling during the polymerization,and then its surface area is increased, and thereby the vaporization ofthe water vapor is promoted, and thereafter the foam is contracted. Inaddition, this form also has an unexpected characteristic such that: thepeeling ability from the polymerization vessel is improved, or the finedivision or disintegration of the hydrogel is made easy.

[0067] A base polymer (water-absorbent resin before surface-crosslinkingtreatment) can be obtained by drying and pulverizing the above hydrogel.

[0068] When the base polymer as obtained is observed with a microscope,the bubble size is comparatively large even in the case where thepolymerization accompanies the foaming. Therefore, the major proportionof particles is in a noncrystalline form containing no bubble.

[0069] In the present invention production process, thesurface-crosslinking treatment of the base polymer may further becarried out, and thereby a water-absorbent resin having a largeabsorption capacity under a load can be obtained. In thesurface-crosslinking treatment, usable are publicly knownsurface-crosslinking agents and publicly known surface-crosslinkingmethods that are usually used for the above uses.

[0070] Incidentally, in the present specification, the terms such ashydrogel, base polymer, and surface-crosslinked water-absorbent resinare used, but any one is a term that represents one form ofwater-absorbent resins.

[0071] The present invention production process has one greatcharacteristic such that the finely divided hydrogel having a high solidcomponent concentration of 55 to 82 weight % as discharged from thepolymerization vessel can be introduced into dryers without using suchas other disintegrators. However, when the occasion demands, the finelydivided hydrogel as obtained by the present invention production processcan also be disintegrated to further make the drying efficiency andpulverization step easy. Then, the disintegration method comes intoquestion. In the case of the water-absorbent resin of an acrylic acid(salt) type, the hydrogel can easily be disintegrated with such as ameat-chopper-type disintegrator when the hydrogel has a solid componentconcentration of less than 55 weight %. In addition, when the solidcomponent concentration is more than 82 weight %, the hydrogel caneasily be pulverized with such as a conventional shock-type pulverizerin the same way as of a dried hydrogel. However, the hydrogel having asolid component concentration of 55 to 82 weight % is difficult tohandle because of its properties, and therefore an attempt toindustrially disintegrate the hydrogel has not been successful yet.

[0072] Accordingly, the present inventors diligently studied how thehydrogel having a high solid component concentration of 55 to 82 weight% formable in the polymerization can be disintegrated. As a result, theyhave found out that the fine division can easily be carried out withspecific disintegrators and pulverizers (these are represented by theterm “disintegrators” in the present patent application.).

[0073] Incidentally, there is no especial limitation on the shape of thehydrogel having a solid component concentration of 55 to 82 weight % assubjected to the disintegration, but the weight-average particlediameter of the hydrogel is favorably not larger than 5 cm, morefavorably not larger than 3 cm.

[0074] A disintegrator having a screen is favorable as an apparatus fordisintegrating the hydrogel having a solid component concentration of 55to 82 weight % in the present invention. Furthermore, the abovedisintegrator is favorably an apparatus corresponding to shear-typecrushers or cutting-shearing mills as described in “Table 16.4,Classification of pulverizers” of “Chemical Engineering Handbook (sixthedition, edited by the Chemical Engineering Society, Maruzen Co., Ltd.,1999)”. The disintegrator is more favorably an apparatus in which thedisintegration is carried out by shearing with fixed and rotary cutters.The disintegration of the hydrogel having a high solid componentconcentration of 55 to 82 weight % can easily be carried out by thedisintegration with these apparatuses; which has hitherto beendifficult.

[0075] Specific examples of the shear-type crushers and cutting-shearingmills are as follows:

[0076] Saw, Round saw, and Band saw (BAND SAW);

[0077] Vertical pulverizer (VERTICAL CUTTING MILL, Orient Co., Ltd.);

[0078] Rotoplex (ROTOPLEX, Hosokawa Micron Co., Ltd.);

[0079] Turbo cutter (TURBO CUTTER, Turbo Industry Co., Ltd.);

[0080] Turbo grinder (TURBO GRINDER, Turbo Industry Co., Ltd.);

[0081] Tyre shredder (TYRE SHREDDER, Masuno Seisakusho Co., Ltd.);

[0082] Rotary cutter mill (ROTARY CUTTER MILL, Yoshida Seisakusho Co.,Ltd.);

[0083] Cutter mill (CUTTER MILL, Tokyo Atomizer Production Co., Ltd.);

[0084] Shred crusher (SHRED CRUSHER, Tokyo Atomizer Production Co.,Ltd.);

[0085] Cutter mill (CUTTER MILL, Masuko Sangyo Co., Ltd.);

[0086] Crusher (CRUSHER, Masuko Sangyo Co., Ltd.);

[0087] Rotary cutter mill (ROTARY CUTTER MILL, Nara Kikai SeisakushoCo., Ltd.);

[0088] Gainax crusher (GAINAX CRUSHER, Horai Co., Ltd.);

[0089] U-com (U-COM, Horai Co., Ltd.); and

[0090] Meshmill (MESHMILL, Horai Co., Ltd.).

[0091] In the present invention, it has been found out that: when thehydrogel having a solid component concentration of 55 to 82 weight % isdisintegrated with a disintegrator, such the hydrogel difficult todisintegrate is disintegrated even with disintegrators except forcutting-type ones, by either one or both of favorably increasing thesolid component concentration by not less than 2 weight % (e.g. if thesolid component concentration is 72 weight % after disintegrating ahydrogel having a solid component concentration of 70 weight %, thesolid component concentration is increased by 2 weight %.) and blowinggas (favorably dry air) into the disintegrator.

[0092] As to the increase of the solid component concentration, as theincreasing ratio rises higher than 2 weight % (e.g. 3 weight % or 4weight %), or as the quantity of the wind as blown is larger, thedisintegration is more easily carried out. However, they should beselected in consideration of economy. The water vapor as generated fromthe hydrogel is condensed in the apparatus during the disintegration,and thereby easily causes adhesion and clogging of the hydrogel in theapparatus. However, it is thought difficult to cause such a phenomenonby the ventilation.

[0093] In addition, when the disintegration is carried out, surfactantsas mentioned in a gazette of JP-A-188726/1999 (Nippon Shokubai Co.,Ltd.) may be added thereto. However, the higher the solid componentconcentration of the hydrogel is, the lower its necessity is.

[0094] The weight-average particle diameter of the hydrogel asdisintegrated by the present invention disintegration means is favorablynot larger than 100 mm, more favorably not larger than 10 mm, still morefavorably not larger than 3 mm, most favorably not larger than 1 mm. Itis ideal that the disintegration can be carried out in the form ofhydrogel until obtaining particles having particle diameters of finalproducts.

[0095] There is no especial limitation on the amount of the residualmonomer of: the finely divided hydrogel as discharged from thepolymerization vessel in the present invention; and the hydrogel asobtained by disintegrating the above hydrogel by the present inventiondisintegration means, but the amount of the residual monomer isfavorably not larger than 3,000 weight ppm for inhibiting scatter of theresidual monomer in such as drying of a following step. Depending uponits use, the amount of the residual monomer is favorably not larger than1,000 weight ppm, more favorably not larger than 500 weight ppm, mostfavorably not larger than 300 weight ppm. In the case where the hydrogelitself is particularly used for uses of sanitary articles such asdisposable diapers, the amount of the residual monomer is favorably notlarger than 1,000 weight ppm, more favorably not larger than 500 weightppm.

[0096] The finely divided hydrogel as discharged from the polymerizationvessel in the present invention, and the hydrogel as obtained bydisintegrating the above hydrogel by the present inventiondisintegration means favorably have a solid component concentration of55 to 82 weight %, an amount of the residual monomer of not larger than1,000 weight ppm, and a weight-average particle diameter of not largerthan 3 mm.

[0097] Incidentally, the present invention disintegrated hydrogel havinga solid component concentration of 55 to 82 weight %, an amount of theresidual monomer of not larger than 1,000 weight ppm, and aweight-average particle diameter of not larger than 3 mm does notinclude products as obtained by adding water to a hydrogel that is onceproduced in a dried state (having a solid component concentration of notless than 83 weight %).

[0098] In the present invention production process, the hydrogel afterthe disintegration may be dried. There is no especial limitation on thedrying method. The drying may be carried out in such a manner thatmaterials are not moved like a belt-type drying method, but favorableused is a drying method which involves moving materials and furthercontacting them with such as hot wind and heated faces sufficiently,like such as an agitation drying method, a fluidized-bed drying method,and a gas-flowing drying method.

[0099] In the present invention production process, the handling of thehydrogel after being disintegrated can be selected from the followingmethods:

[0100] (1) The hydrogel, as it is, is obtained as a product.; Thehydrogel, as it is, is subjected to uses such as sanitary articles andagricultural and horticultural articles. Fine particulate inorganicsubstances (e.g. bentonite, zeolite, and silicon oxide) may be addedthereto for the fluidity of particles.

[0101] (2) The surface-crosslinking agent mixes and reacts with thehydrogel, and the resultant mixture in a state of containing water isobtained as a product.; The energy for evaporating water is unnecessary,Fine particulate inorganic substances (e.g. bentonite, zeolite, andsilicon oxide) may be added thereto for the fluidity of particles.

[0102] (3) The surface-crosslinking agent mixes and reacts with thehydrogel, and the resultant mixture is dried, thus obtaining a product.;The heat energy for the drying can combine the energy for thesurface-crosslinking reaction.

[0103] (4) The hydrogel is dried, and the resultant dried product, as itis, is obtained as a product.

[0104] (5) The hydrogel is dried, and then the resultant dried productis pulverized and classified, thus obtaining a product.

[0105] (6) The hydrogel is dried, and then the resultant dried productis pulverized, classified and surface-crosslinked, thus obtaining aproduct.

[0106] The hydrogel having a solid component concentration of 55 to 82weight %, which has hitherto been difficult to disintegrate, is obtainedas a finely divided hydrogel. Therefore, it newly has enabled thefollowing:

[0107] 1) The above methods (1), (2), and (3) can be carried out

[0108] 2) Adoptable is a drying method that involves moving materialsand further contacting them with such as hot wind and heated facessufficiently, like such as an agitation drying method, a fluidized-beddrying method, and a gas-flowing drying method having good heatefficiency, which methods have been difficult to use as drying methodsunless materials having mold-releaseability (e.g. surfactants) are notadded thereto in the drying of the hydrogel having a solid componentconcentration of less than 55 weight %.

[0109] 3) The disintegration of the polymer can be carried out in astate of containing water, and therefore a fine powder is difficult tocause, and a particulate hydrogel having little fine powder is obtained.

[0110] The water-absorbent resin as surface-crosslinked in the presentinvention favorably displays an absorption capacity of not less than 20(g/g) under a load (AAP), more favorably not less than 30 (g/g), stillmore favorably not less than 35 g/g). In the case where the absorptioncapacity is less than 20 (g/g), there are disadvantages in that thefavorable performance is not displayed when the water-absorbent resin asobtained is used as a sanitary article.

[0111] Various functions can also be given by adding, to the presentinvention water-absorbent resin, such as disinfectants, antimicrobialagents, perfumes, various inorganic powders, foaming agents, pigments,dyes, hydrophilic short fibers, manure, oxidants, reductants, water, andsalts, in an amount of favorably not larger than 20 parts by weight,more favorably not larger than 10 parts by weight, in the productionprocess or after the production. Favorable examples of the compounds asadded include water-insoluble inorganic powders, and/or polyamines.

[0112] The present invention process enables easy production of thewater-absorbent resin, which has good absorption properties that areexcellent in balance of the absorption capacity without load, absorptioncapacity under a load, and extractable content. The water-absorbentresin is widely used for water-retaining agents in agricultural andhorticultural fields, water-holding materials in engineering worksfields, hygroscopic agents, moisture-removing agents, and buildingmaterials, but the present invention water-absorbent resin isparticularly favorably used for sanitary materials such as disposablediapers, incontinent pads, mother's breast pads, and sanitary napkins.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0113] Hereinafter, the present invention is more specificallyillustrated by the following examples and comparative examples. However,the present invention is not limited to these examples. Incidentally, inthe examples, unless otherwise noted, the unit “part(s)” denotes“part(s) by weight”.

[0114] [Measurement of Absorption Capacity Without Load (GV)]:

[0115] To a nonwoven-fabric-made bag (60 mm×60 mm), 0.2 g ofwater-absorbent resin was uniformly added, and then immersed in a 0.9weight % aqueous sodium chloride solution (physiological saline). Thebag was pulled up after 30 minutes, and the weight W1 (g) of the bag wasmeasured after draining off at 250×9.81 m/sec² (250 G) for 3 minuteswith a centrifugal separator. The same procedure was carried out withoutusing the water-absorbent resin, and then the weight WO (g) of the bagwas measured. Then, the GV (absorption capacity without load) wascalculated from these weights W1 and W0 in accordance with the followingequation:

GV(g/g)={(weight W1 (g)−weight W0(g))/weight of water-absorbent resin(g)−1 }

[0116] [Extractable content]:

[0117] Into a plastic receptacle of 250 ml in capacity having a lid,184.3 g of 0.9 weight % aqueous NaCl solution (physiological saline) wasweighed out. Then, 1.00 g of water-absorbent resin was added to theaqueous solution, and they were stirred for 16 hours, thereby theextractable content in the resin was extracted. This extract liquid wasfiltrated with a filter paper, and then 50.0 g of the resultant filtratewas weighed out and regarded as a measuring solution.

[0118] To begin with, only the physiological saline was firstly titratedwith an aqueous 0.1N NaOH solution until the pH reached 10, and then theresultant solution was titrated with an aqueous 0.1N HCl solution untilthe pH reached 2.7, thus obtaining blank titration amounts ([bNaOH] mland [bHCl] ml).

[0119] The same titration procedure was carried out for the measuringsolution, thus obtaining titration amounts ([NaOH] ml and [HCl] ml).

[0120] For example, if the water-absorbent resin comprises acrylic acidand its sodium salt, the extractable content and neutralization ratio ofthe water-absorbent resin can be calculated from their weight-averagemolecular weights and the titration amounts as obtained from the aboveprocedure, in accordance with the following equation.

neutralization ratio (mol %)=

{1−([NaOH]−[bNaOH])/([HCl]−[bHCl])}100

extractable content (weight %)=

0.1×Mw×184.3×100×([HCl]−[bHCl])/1,000/1.0/150.0

(where: Mw=72.06×(1−neutralization ratio/100)+94.04×neutralizationratio/100)

[0121] [Measurement of Residual Monomer):

[0122] To 1,000 g of deionized water, 0.5 g of water-absorbent resin wasadded, and extracted for 2 hours under agitation. Thereafter, theresultant swollen-gelled water-absorbent resin was filtrated with afilter paper, and the amount of the residual monomer in the resultantfiltrate was analyzed with liquid chromatography. On the one hand, thecalibration curve as obtained by analyzing a standard monomer solutionhaving a known concentration in the same way was used as an externalstandard, and the amount of the residual monomer in the water-absorbentresin was determined in consideration of the dilution ratio of thefiltrate.

[0123] In addition, the measurement of the residual monomer in thehydrogel was carried out in the same way as of the water-absorbent resinexcept that: the hydrogel was used in an amount of 0.5 g in terms ofsolid content, and the immersing time was 24 hours in the deionizedwater, and the solid content correction was carried out when theresidual monomer was calculated.

[0124] [Measurement of Solid Content Concentration of Hydrogel]:

[0125] A small amount of a portion of hydrogel as taken out from apolymerization vessel was cut away and quickly cooled, and quicklyfinely divided with scissors. Then, the solid content concentration wascalculated by putting 5 g of the resultant hydrogel in a petri dish andthen drying in a dryer of 180° C. for 24 hours. The solid contentconcentration of a particulate hydrogel was calculated by putting 5 g ofa sample in a petri dish and then drying in a dryer of 180° C. for 24hours.

[0126] [Calculation of Concentration Ratio]:

[0127] The ratio (concentration ratio) is a ratio of a solid componentconcentration of a hydrogel as formed by. polymerization to a solidcomponent concentration in an aqueous solution of a monomer component.Herein, the solid component in the aqueous solution of the monomercomponent means a monomer and other additives, and does not includewater and solvents. For example, when the solid component concentrationin the aqueous solution of the monomer component is 40 weight %, and thesolid component concentration of the hydrogel as formed is 48 weight %,the concentration ratio is calculated as follows: 48/40=1.20.

[0128] [Measurement of Absorption Capacity Under Load (AAP)]:

[0129] An amount of 0.9 g of water-absorbent resin was uniformly spreadon a stainless wire net of 400 mesh (mesh opening size: 38 μm) asattached by fusion to the bottom of a plastic supporting cylinder of aninner diameter 60 mm, on which a piston and a load were further mountedin sequence, wherein: the total weight was adjusted to 565 g so that aload of 20 g/cm² (corresponding to 1.96 kPa) could be uniformly appliedto the water-absorbent resin, and the piston had an outer diameter onlya little smaller than 60 mm and made no gap with the wall face of thesupporting cylinder, but was not hindered from moving up and down. Then,the weight (Wa) of the resultant set of measurement apparatus wasmeasured.

[0130] A glass filter having a diameter of 90 mm was mounted inside aPetri dish having a diameter of 150 mm, and a 0.9 weight % aqueous NaClsolution was added up to the same level as the upper surface of theglass filter, on which a filter paper having a diameter of 90 mm wasthen mounted such that its entire surface would be wet, and theexcessive liquid was removed.

[0131] The above set of measurement apparatus was mounted on the abovewet filter paper, thereby allowing to absorb the liquid under a load.After 1 hour, the set of measurement apparatus was lifted and removed,and its weight (Wb) was measured again.

[0132] The absorption capacity under a load (AAP) is calculated inaccordance with the following equation:

AAP(g/g)=(Wb−Wa)/0.9

[0133] [Measurement of Temperature of Polymerization System]:

[0134] For measuring the temperature of the system of which thetemperature was rapidly changed, a deta-collecting system of PC cardtype NR-1000 (made by Keyence Co., Ltd.) was used, and the temperaturewas measured with a sampling cycle of 0.1 second by installing athermocouple in the polymerization system. From the resultanttemperature-time chart, the polymerization initiation temperature andthe peak temperature (highest temperature) were read.

[0135] [Polymerization Time]:

[0136] Measured was how long time passed since the aqueous solution ofthe monomer component was added to the polymerization vessel until thepeak temperature. Specifically, the following was measured: (inductionperiod)+(how long time passed since the initiation of the polymerizationuntil the peak temperature was reached).

EXAMPLE 1

[0137] A mixed solution was continuously supplied into a continuouskneader (made by Dalton Co., Ltd., CKDJS40) as a polymerization vesselhaving two agitation paddles, wherein the mixed solution was obtained byline-mixing the following components with amounts per minute: 493.2 g ofacrylic acid, 396.1 g of 48 weight % aqueous sodium hydoxide solution,419.6 g of water, 6.0 g of 0.5 weight % aqueous solution of diethylenetriamine pentaacetic acid pentasodium salt, 1.0 g of polyethylene glycoldiacrylate (average polyethylene glycol unit number: 8) as aninternal-crosslinking agent, and 11.3 g of 3 weight % aqueous sodiumpersulfate solution. Incidentally, the concentration of the aqueoussolution of the water-soluble unsaturated monomer component was 45weight %, and the temperature of the mixed solution as supplied to thepolymerization vessel reached 97° C. by the heat of neutralization andheat of dissolution. In addition, the jacket temperature of the abovepolymerization vessel was adjusted to 100° C., and nitrogen gas wasblown into the polymerization vessel at a flow rate of 20 L/minute.

[0138] The polymerization was initiated as soon as the above mixedsolution was supplied into the polymerization vessel, and the resultantcrosslinked hydrogel polymer was sheared while the polymerization wascarried out, and the resultant crushed hydrogel (1) was continuouslydischarged from the polymerization vessel. In addition, the peaktemperature of the reaction system was then 101.2° C.

[0139] The hydrogel (1) as obtained in this way was spread on a metalgauze with a mesh opening size of 850μm to form a layer of about 50 mmin thickness. Subsequently, the hydrogel was hot-wind-dried by passing ahot wind having a temperature of 170° C. (dew point: 50° C.) at a speedof 1 m/sec through the hydrogel in its vertical direction for 40minutes. Pulverized was a block-shaped dried material which was obtainedin this way and comprised of a particulate dry polymer, and further theresultant pulverized product was classified with a JIS standard sievehaving a mesh opening size of 850 μm, thus obtaining a water-absorbentresin powder (1).

[0140] The results are listed in Table 1.

EXAMPLE 2

[0141] A water-absorbent resin powder (2) was obtained in the same wayas of Example 1 except for being changed as follows: 435.7 g of acrylicacid, 349.9 g of 48 weight % aqueous sodium hydoxide solution, 193.3 gof water, 5.3 g of 0.5 weight % aqueous solution of diethylene triaminepentaacetic acid pentasodium salt, 0.44 g of polyethylene glycoldiacrylate (average polyethylene glycol unit number: 8) as aninternal-crosslinking agent, and 10.1 g of 3 weight % aqueous sodiumpersulfate solution. Incidentally, the concentration of the aqueoussolution of the water-soluble unsaturated monomer component was then 53weight %, and the temperature of the above aqueous solution as suppliedto the polymerization vessel was 99° C., and the peak temperature of thereaction system was 102.1° C.

[0142] The results are listed in Table 1.

EXAMPLE 3

[0143] A water-absorbent resin powder (3) was obtained in the same wayas of Example 1 except for being changed as follows: 438.4 g of acrylicacid, 352.1 g of 48 weight % aqueous sodium hydoxide solution, 520.5 gof water, 5.3 g of 0.5 weight % aqueous solution of diethylene triaminepentaacetic acid pentasodium salt, 1.48 g of polyethylene glycoldiacrylate (average polyethylene glycol unit number: 8) as anintemal-crosslinking agent, and 10.1 g of 3 weight % aqueous sodiumpersulfate solution. Incidentally, the concentration of the aqueoussolution of the water-soluble unsaturated monomer component was then 40weight %, and the temperature of the above aqueous solution as suppliedto the polymerization vessel was 92° C., and the peak temperature of thereaction system was 100.5° C.

[0144] The results are listed in Table 1.

EXAMPLE 4

[0145] A water-absorbent resin powder (4) was obtained in the same wayas of Example 1 except that: 0.053 g per minute of2-hydroxy-2-methyl-1-phenyl-propane-1-one was added to the aqueoussolution of the water-soluble unsaturated monomer component asline-mixed, and further a black-light mercury lamp (peak wave length:352 nm, form: H400BL; as provided in.. a searchlight projector (MT4020);and both made by Toshiba Lighting & Technology Corporation) was set on aceiling plate of the continuous kneader, and the UV-radiation wascarried out. Incidentally, the temperature of the aqueous solution ofthe water-soluble unsaturated monomer component as supplied to thepolymerization vessel was then 97° C., and the peak temperature of thereaction system was 102.0° C.

[0146] The results are listed in Table 1.

EXAMPLE 5

[0147] A water-absorbent resin powder (5) was obtained in the same wayas of Example 1 except that: the water-absorbent resin powder (1) asobtained in Example 1 was classified with a JIS standard sieve having amesh opening size of 150 μm, and 60 g per minute of the resultant finepowder was continuously supplied into the polymerization vessel.Incidentally, the peak temperature of the reaction system was 101.8° C.

[0148] The results are listed in Table 1.

EXAMPLE 6

[0149] Prepared was an aqueous solution of a water-soluble unsaturatedmonomer component, including 162.7 g of acrylic acid, 1,722.3 g of 37weight % aqueous sodium acrylate solution, and 1.75 g of polyethyleneglycol diacrylate (average polyethylene glycol unit number: 8) as aninternal-crosslinking agent. Subsequently, the replacement with nitrogenwas carried out for the above aqueous solution of the monomer componentfor 30 minutes. Thereafter, 90.3 g of 1 weight % aqueous sodiumpersulfate solution and 22.6 g of 0.2 weight % aqueous L-ascorbic acidsolution were added to the above aqueous solution of the monomercomponent, and at the same time the resultant mixture was warmed througha SUS tube immersed in an oil bath of 65° C., and then the mixture wassupplied into a polymerization vessel causing shearing action.Incidentally, a jacketed stainless twin-arm kneader of 10 liters incapacity with two sigma-type blades was used as the above polymerizationvessel, and the jacket temperature of the polymerization vessel wasraised to 95° C. by passing warm water, and the above aqueous solutionwas supplied while the blades were rotated. Incidentally, theconcentration of the aqueous solution of the monomer component was then40 weight %. The polymerization was initiated after seconds from thesupply of the above aqueous solution, and the resultant crosslinkedhydrogel polymer was sheared while the polymerization was carried out.After 1 minute, the reaction system reached its peak temperature of 102°C., and then the polymerization was completed after 10 minutes since thepeak temperature was shown.

[0150] A hydrogel (6) as obtained in this way was dried, pulverized, andclassified in the same way as of Example 1, thus obtaining awater-absorbent resin powder (6).

[0151] The results are listed in Table 1.

EXAMPLE 7

[0152] Prepared was an aqueous solution of a water-soluble unsaturatedmonomer component, including 162.7 g of acrylic acid, 1,722.3 g of 37weight % aqueous sodium acrylate solution, and 1.75 g of polyethyleneglycol diacrylate (average polyethylene glycol unit number: 8) as aninternal-crosslinking agent. Subsequently, the replacement with nitrogenwas carried out for the above aqueous solution of the monomer componentfor 30 minutes. Thereafter, 90.3 g of 1 weight % aqueous sodiumpersulfate solution and 22.6 g of 0.2 weight % aqueous L-ascorbic acidsolution were added to the above aqueous solution of the monomercomponent, and at the same time the resultant mixture was warmed througha SUS tube immersed in an oil bath of 55° C., and then the mixture wassupplied into a polymerization vessel causing the same (condition of)shearing action as of Example 6. Incidentally, the concentration of theaqueous solution of the monomer component was then 40 weight %. Thepolymerization was initiated after 17 seconds from the supply of theabove aqueous solution, and the resultant crosslinked hydrogel polymerwas sheared while the polymerization was carried out. After 1.5 minutes,the reaction system reached its peak temperature of 103.5° C., and thenthe polymerization was completed after 10 minutes since the peaktemperature was shown.

[0153] A hydrogel (7) as obtained in this way was dried, pulverized, andclassified in the same way as of Example 1, thus obtaining awater-absorbent resin powder (7).

[0154] The results are listed in Table 1.

EXAMPLE 8

[0155] Prepared was an aqueous solution of a water-soluble unsaturatedmonomer component by blending: a solution as obtained by dissolving 0.42g of polyethylene glycol diacrylate (average polyethylene glycol unitnumber: 8) as an internal-crosslinking agent into 618 g of acrylic acid;495 g of 48.5 weight % aqueous sodium hydroxide solution; and 335 g ofwater, at a stroke. Subsequently, the replacement with nitrogen wascarried out for the above aqueous solution of the monomer component for30 minutes. Thereafter, 51.5 g of 0.5 weight % aqueous sodium persulfatesolution was added to the above aqueous solution of the monomercomponent, and at the same time the resultant mixture was warmed througha SUS tube immersed in an oil bath of 96° C., and then the mixture wassupplied into a polymerization vessel causing the same (condition of)shearing action as of Example 6. Incidentally, the aqueous solution ofthe monomer component then had a concentration of 50 weight %, and aneutralization ratio of 70 mol %. The polymerization was initiated afterS seconds from the supply of the above aqueous solution, and theresultant crosslinked hydrogel polymer was sheared while thepolymerization was carried out. After 1 minute, the reaction systemreached its peak temperature of 102.8° C., and then the polymerizationwas completed after 3 minutes since the peak temperature was shown.

[0156] A hydrogel (8) as obtained in this way was pulverized with avertical pulverizer (type: VM27-S, made by Orient Co., Ltd., diameter ofscreen: 8 mm) in order to obtain a particulate hydrogel, and theparticulate hydrogel was spread on a metal gauze with a mesh openingsize of 850 μm to form a layer of about 50 mm in thickness.Subsequently, the hydrogel was hot-wind-dried by passing a hot windhaving a temperature of 170° C. (dew point: 50° C.) at a speed of 1m/sec through the hydrogel in its vertical direction for 20 minutes.Pulverized was a block-shaped dried material which was obtained in thisway and comprised of a particulate dry polymer, and further theresultant pulverized product was classified with a JIS standard sievehaving a mesh opening size of 850 μm, thus obtaining a water-absorbentresin powder (8).

[0157] The results are listed in Table 1.

EXAMPLE 9

[0158] A water-absorbent resin powder (9) was obtained in the same wayas of Example 8 except for using water obtained by cooling and recyclingvapor in the polymerization vessel in Example 1 as the 335 g of water inthe aqueous solution of the water-soluble unsaturated monomer componentin Example 8. Incidentally, the peak temperature of the reaction systemwas then 103.3° C.

[0159] The results are listed in Table 1.

EXAMPLE 10

[0160] To 500 g of the water-absorbent resin powder (1) as obtained inExample 1, an aqueous surface-crosslinking agent solution including1,4-butanediol, propylene glycol, and water in amounts of 0.32 weight %,0.5 weight %, and 2.73 weight % (relative to the powder) respectivelywas added, and the resultant mixture was heated for 30 minutes in amixer as heated in an oil bath of 212° C., thus obtaining asurface-crosslinked water-absorbent resin powder (10).

[0161] The result is listed in Table 2.

EXAMPLE 11

[0162] To 500 g of the water-absorbent resin powder (2) as obtained inExample 2, an aqueous surface-crosslinking agent solution including1,4-butanediol, propylene glycol, and water in amounts of 0.32 weight %,0.5 weight %, and 2.73 weight % (relative to the powder) respectivelywas added, and the resultant mixture was heated for 25 minutes in amixer as heated in an oil bath of 212° C., thus obtaining asurface-crosslinked water-absorbent resin powder (11).

[0163] The result is listed in Table 2.

EXAMPLE 12

[0164] To 500 g of the water-absorbent resin powder (3) as obtained inExample 3, an aqueous surface-crosslinking agent solution including1,4butanediol, propylene glycol, and water in amounts of 0.32 weight %,0.5 weight %, and 2.73 weight % (relative to the powder) respectivelywas added, and the resultant mixture was heated for 25 minutes in amixer as heated in an oil bath of 212° C., thus obtaining asurface-crosslinked water-absorbent resin powder (12).

[0165] The result is listed in Table 2.

EXAMPLE 13

[0166] To 500 g of the water-absorbent resin powder (6) as obtained inExample 6, an aqueous surface-crosslinking agent solution including1,4-butanediol, propylene glycol, and water in amounts of 0.32 weight %,0.5 weight %, and 2.73 weight % (relative to the powder) respectivelywas added, and the resultant mixture was heat-stirred for 35 minutes ina mixer as heated in an oil bath of 212° C., thus obtaining asurface-crosslinked water-absorbent resin powder (13).

[0167] The result is listed in Table 2.

[0168] [Comparative Example 1]

[0169] Prepared was an aqueous solution of a water-soluble unsaturatedmonomer component with a concentration of 41 weight %, including 633.4 gof acrylic acid, 5550.6 g of 37 weight % aqueous sodium acrylatesolution, 3.72 g of polyethylene glycol diacrylate (average polyethyleneglycol unit number: 8) as an internal-crosslinking agent, and 342.4 g ofwater.

[0170] The above aqueous solution of the water-soluble unsaturatedmonomer component was supplied to the polymerization vessel of Example1, and the atmosphere in the polymerization vessel was replaced withnitrogen for 30 minutes while the above aqueous solution was maintainedat 25° C. Subsequently, while the jacket temperature of thepolymerization vessel was adjusted to 25° C. under a stream of nitrogenand the blades were rotated, 24.5 g of 15 weight % aqueous sodiumpersulfate solution and 15.3 g of 0.2 weight % aqueous L-ascorbic acidsolution were added thereto. As a result, the polymerization wasinitiated after 10 seconds. The warm water of the jacket was heated to70° C. at the same time as the initiation of the polymerization, and theresultant crosslinked hydrogel polymer was sheared while thepolymerization was carried out. After 6.5 minutes, the reaction systemreached its peak temperature, and then the polymerization was completedafter 20 minutes since the peak temperature was shown.

[0171] A comparative hydrogel (1) as obtained in this way was spread ona metal gauze with a mesh opening size of 850 μm to form a layer ofabout 50 mm in thickness. Subsequently, the hydrogel was hot-wind-driedby passing a hot wind having a temperature of 170° C. (dew point: 50°C.) at a speed of 1 m/sec through the hydrogel in its vertical directionfor 60 minutes. Pulverized was a block-shaped dried material which wasobtained in this way and comprised of a particulate dry polymer, andfurther the resultant pulverized product was classified with a JISstandard sieve having a mesh opening size of 850 μm, thus obtaining acomparative water-absorbent resin powder (1).

[0172] The results are listed in Table 1.

[0173] [Comparative Example 2]

[0174] A comparative water-absorbent resin powder (2) was obtained inthe same way as of Comparative Example 1 except for being changed asfollows: 908.5 g of acrylic acid, 4,807.1 g of 37 weight % aqueoussodium acrylate solution, 3.83 g of polyethylene glycol diacrylate(average polyethylene glycol unit number: 8) as an internal-crosslinkingagent, 809.6 g of water, 25.2 g of 15 weight % aqueous sodium persulfatesolution, and 15.8 g of 0.2 weight % aqueous L-ascorbic acid solution.

[0175] The results are listed in Table 1.

[0176] [Comparative Example 3]

[0177] Prepared was an aqueous solution of a water-soluble unsaturatedmonomer component with a concentration of 41 weight %, including 192.8 gof acrylic acid, 1689.7 g of 37 weight % aqueous sodium acrylatesolution, 1.14 g of polyethylene glycol diacrylate (average polyethyleneglycol unit number: 8) as an internal-crosslinking agent, and 104.2 g ofwater.

[0178] The replacement with nitrogen was carried out for the aboveaqueous solution of the water-soluble unsaturated monomer component for30 minutes, and the aqueous solution was supplied to the polymerizationvessel of Example 5, and the jacket temperature was adjusted to 50° C.,and then the above aqueous solution of the water-soluble unsaturatedmonomer component was warmed to 50° C. Subsequently, while the bladeswere rotated under a stream of nitrogen, 7.5 g of 15 weight % aqueoussodium persulfate solution and 4.7 g of 0.2 weight % aqueous L-ascorbicacid solution were added thereto. As a result, the polymerization wasinitiated after 15 seconds. The warm water of the jacket was heated to70° C. at the same time as the initiation of the polymerization, and theresultant crosslinked hydrogel polymer was sheared while thepolymerization was carried out After 1.5 minutes, the reaction systemreached its peak temperature, and then the polymerization was completedafter 10 minutes since the peak temperature was shown.

[0179] A comparative hydrogel (3) as obtained in this way washot-wind-dried in the same way as of Example 1. Pulverized was ablock-shaped dried material which was obtained in this way and comprisedof a particulate dry polymer, and further the resultant pulverizedproduct was classified with a JIS standard sieve having a mesh openingsize of 850 μm, thus obtaining a comparative water-absorbent resinpowder (3).

[0180] The results are listed in Table 1.

[0181] [Comparative Example 4]

[0182] To 500 g of the comparative water-absorbent resin powder (1) asobtained in Comparative Example 1, an aqueous surface-crosslinking agentsolution including 1,4butanediol, propylene glycol, and water in amountsof 0.32 weight %, 0.5 weight %, and 2.73 weight % (relative to thepowder) respectively was added, and the resultant mixture was heated for45 minutes in a mixer as heated in an oil bath of 212° C., thusobtaining a comparative surface-crosslinked water-absorbent resin powder(4).

[0183] The result is listed in Table 2.

[0184] [Comparative Example 5]

[0185] To 500 g of the comparative water-absorbent resin powder (2) asobtained in Comparative Example 2, an aqueous surface-crosslinking agentsolution including 1,4-butanediol, propylene glycol, and water inamounts of 0.32 weight %, 0.5 weight %, and 2.73 weight % (relative tothe powder) respectively was added, and the resultant mixture was heatedfor 30 minutes in a mixer as heated in an oil bath of 212° C., thusobtaining a comparative surface-crosslinked water-absorbent resin powder(5).

[0186] The result is listed in Table 2. TABLE 1 Concentration SolidIncrease of solid Amount of of component component residual aqueousSolid component Neutralization concentration concentration ofExtractable monomer monomer concentration of ratio of Concen- ofparticulate hydrogel during GV content (weight solution hydrogelhydrogel tration hydrogel disintegration (g/g) (weight %) ppm) (weight%) (weight %) (%) ratio (weight %) (weight %) Water-absorbent 39.0 12.0840 45 57.5 — 1.278 — — resin powder (1) Water-absorbent 42.8 16.6 57053 66.3 — 1.251 — — resin powder (2) Water-absorbent 44.1 10.3 980 4051.2 — 1.280 — — resin powder (3) Water-absorbent 38.7  9.9 310 45 58.2— 1.293 — — resin powder (4) Water-absorbent 38.1 13.2 920 45 61.3 — — —— resin powder (5) Water-absorbent 44.5 14.6 820 40 46.6 — 1.165 — —resin powder (6) Water-absorbent 47.5 14.5 380 40 45.9 — 1.148 — — resinpowder (7) Water-absorbent 49.6 18.2 730 50 62.3 74.6 1.246 64.9 2.6resin powder (8) Water-absorbent 49.1 17.7 760 50 61.7 — 1.234 — — resinpowder (9) Comparative 45.9 20.6 310 41 42.7 — 1.041 — — water-absorbentresin powder (1) Comparative 47.7 26.5 110 41 41.9 — 1.022 — —water-absorbent resin powder (2) Comparative 48.9 24.5 580 41 44.8 —1.093 — — water-absorbent resin powder (3)

[0187] As being understood from the results of Table 1, awater-absorbent resin (base polymer) was obtained with high productivityby carrying out the present invention, wherein the water-absorbent resindisplayed high absorption capacity without load (GV) and had a smallextractable content. TABLE 2 AAP (g/g) Water-absorbent resin powder 31.3(10) Water-absorbent resin powder 34.1 (11) Water-absorbent resin powder34.8 (12) Water-absorbent resin powder 35.0 (13) Comparativewater-absorbent 32.5 resin powder (4) Comparative water-absorbent 28.8resin powder (5)

[0188] As being understood from the results of Table 2, awater-absorbent resin that displayed high absorption capacity under aload (AAP) could be obtained also among the surface-crosslinking-treatedwater-absorbent resins.

INDUSTRIAL APPLICATION

[0189] The present invention can provide: a base polymer, which displayshigh absorption capacity without load and has a small extractablecontent; and a water-absorbent resin, which issurface-crosslinking-treated and displays high absorption capacity undera load, by reasonable steps.

[0190] The above effects are obtained, and therefore the water-absorbentresin as obtained by the present invention is useful for such as: usescontacting with human bodies, (e.g. sanitary articles, such asdisposable diapers for child and adult, sanitary napkins, andincontinent articles for adult); water-retaining agents for plant orsoil, water-holding materials for electric wire or photocable; andwater-holding materials in engineering and construction works fields.

1. A production process for a water-absorbent resin, which comprises apolymerization step that includes the steps of: supplying an aqueoussolution of a water-soluble unsaturated monomer component including amajor proportion of acrylic acid and/or its salt into a polymerizationvessel causing shearing action; and then carrying out polymerization,involving crosslinking, of the water-soluble unsaturated monomer and atthe same time carrying out fine division of the resultant hydrogel, withthe production process being characterized in that the aqueous solutionof the water-soluble unsaturated monomer component as supplied into thepolymerization vessel has a temperature of not lower than 40° C.
 2. Aproduction process according to claim 1, wherein the polymerization ascarried out in the polymerization step is continuous polymerization thatinvolves continuously supplying the aqueous solution of thewater-soluble unsaturated monomer component and continuously dischargingthe resultant hydrogel.
 3. A production process according to claim 1 or2, wherein the water-soluble unsaturated monomer component in theaqueous solution has a concentration of not less than 30 weight %.
 4. Aproduction process according to any one of claims 1 to 3, which involvesa concentration ratio of not less than 1.10 wherein the concentrationratio is defined as a ratio of a solid component concentration of theresultant hydrogel as discharged from the polymerization vessel to asolid component concentration of the aqueous solution of thewater-soluble unsaturated monomer component as supplied into thepolymerization vessel.
 5. A production process according to any one ofclaims 1 to 4, wherein the hydrogel displays the highest temperature ofnot lower than 100° C. in the polymerization vessel.
 6. A productionprocess according to any one of claims 1 to 5, which involvesutilization of heat of neutralization and/or heat of dissolution ofacrylic acid and an alkali for heating to raise the temperature of theaqueous solution of the water-soluble unsaturated monomer component assupplied into the polymerization vessel.
 7. A production processaccording to any one of claims 1 to 6, which further comprises the stepsof collecting and then reusing acrylic acid and/or water havingvaporized during the polymerization.
 8. A production process accordingto any one of claims 1 to 7, which involves a difference AT of nothigher than 70° C. between the temperature of the aqueous solution ofthe water-soluble unsaturated monomer component as supplied into thepolymerization vessel and the highest temperature of the hydrogel in thepolymerization vessel.
 9. A production process according to any one ofclaims 1 to 8, which further comprises the step of adding awater-absorbent resin fine powder either to the aqueous solution of thewater-soluble unsaturated monomer component as supplied into thepolymerization vessel or at the same time as this supply of the aqueoussolution of the water-soluble unsaturated monomer component.
 10. Aproduction process according to any one of claims 1 to 9, whereinacrylic acid containing p-methoxyphenol is used as the water-solubleunsaturated monomer component.
 11. A production process according to anyone of claims 1 to 10, which further comprises the step of carrying outdisintegration of the resultant finely divided hydrogel, as dischargedfrom the polymerization vessel, with a disintegrator having a screen.12. A production process according to claim 11, wherein the finelydivided hydrogel as discharged from the polymerization vessel isdisintegrated with the disintegrator in such a manner that the solidcomponent concentration will increase by not less than 2 weight % duringthe disintegration.
 13. A production process according to any one ofclaims 1 to 12, which further comprises a surface-crosslinking stepafter the polymerization step.
 14. A water-absorbent resin as obtainedby the production process as recited in any one of claims 1 to 13, whichdisplays an absorption capacity of not less than 20 g/g under a load.15. A sanitary article, which comprises the water-absorbent resin asrecited in claim 14.